Step 1: The first step is to open Google Classroom and click Classwork
Step 2: Click Create
Step 3: Click Topic
Step 4: Type the title of your resource topic (section) in Topic
Step 5: Click Add
Step 6: Click Create
Step 7: Click Material
Step 8: Type the title of your resource in Title
Step 9: Click Attach Google Drive item or other media type and follow through those steps to add the resource
Step 10: Click arrow next to Topic
Step 11: Click appropriate option
Step 12: Click Save options
Step 13: Click your choice
Step 14: Click Post
Step 15: That's it. You're done.
Here's an interactive tutorial for the visual learners
https://www.iorad.com/player/143486/Google-Classroom--Adding-Resource-Section